1. Field of the Invention
The present invention relates to an optical transmitter having an external modulator.
2. Description of the Related Art
In an optical fiber communication system, a modulation rate is increasing with an increase in capacity of the system. In direct intensity modulation of a laser diode, wavelength chirping is a problem. The chirping causes waveform distortion when an optical signal passes an optical fiber having chromatic dispersion. From a standpoint of fiber loss, the most desirable wavelength to be applied to a silica fiber is 1.55 xcexcm. At this wavelength, a normal fiber has a chromatic dispersion of about 18 ps/km/nm, which limits a transmission distance. To avoid this problem, an external modulator has increasingly been expected.
As a practical external modulator, a Madh-Zehnder type optical modulator (LN modulator) using LiNbO3 (lithium niobate) as a substrate material has been developed. Continuous-wave light (CW light) having a constant intensity from a light source is supplied to the LN modulator, in which a switching operation using interference of light is carried out to obtain an intensity-modulated optical signal.
The LN modulator has a frequently pointed-out defect that it causes operation point drift. To cope with the operation point drift, light output from the LN modulator is monitored, and control for operation point stabilization is carried out according to an electrical signal obtained as the result of this monitoring.
FIG. 1 is a block diagram showing an optical transmitter in the related art. This optical transmitter includes a laser diode (LD) 2 as a light source for outputting a light beam as CW light, and an external modulator 4 for modulating the light beam output from the laser diode 2 according to a main signal to thereby output an optical signal. A drive current for the laser diode 2 is maintained constant by an LD current control circuit 6, so as to maintain constant the power of the light beam to be output from the laser diode 2. Further, to maintain constant the wavelength of the light beam to be output from the laser diode 2, a temperature sensor such as a thermistor (not shown) is provided in the vicinity of the laser diode 2, and an LD temperature control circuit 8 is provided to perform feedback control such that the temperature detected by the temperature sensor becomes constant.
Throughout the drawings, the thick solid line arrows represent the flow of electrical signals, and the thin solid line arrows represent the flow of optical signals or light beams.
In the conventional optical transmitter shown in FIG. 1, the drive current for the laser diode 2 is maintained constant by the LD current control circuit 6. Accordingly, when the characteristics of the laser diode 2 vary because of aging or the like of the laser diode 2, there is a possibility that the power of the light beam to be output from the laser diode 2 may vary. Another conventional optical transmitter improved in this respect will be described with reference to FIG. 2.
FIG. 2 is a block diagram showing such an improved optical transmitter in the related art. An optical output fixing control or automatic power control (APC) circuit 10 is provided to perform feedback control of the power of a light beam to be output from a laser diode 2. The laser diode 2 outputs a forward beam and a backward beam. The forward beam is supplied to an external modulator 4, and the backward beam is supplied to a back power monitor 12. An LD current control circuit 14 controls a drive current to be supplied to the laser diode 2 so that the power of the backward beam detected by the back power monitor 12 becomes constant. The power of the forward beam is proportional to the power of the backward beam. Accordingly, such feedback control enables the power of the forward beam to be maintained constant.
However, the improved optical transmitter in the related art shown in FIG. 2 yet has a problem such that it is difficult to maintain the output power of an optical signal to be obtained constant with high accuracy. Although the power of the light beam to be output from the laser diode 2 is maintained constant by the feedback control as mentioned above, there is a possibility of variations in loss by the external modulator 4 due to changes in outside environments such as temperature, resulting in variations in the power of an optical signal to be output from the external modulator 4.
It is therefore an object of the present invention to provide an optical transmitter having an external modulator wherein the output power of an optical signal to be output from the optical transmitter can be maintained constant with high accuracy.
It is another object of the present invention to provide an optical transmitter having an external modulator wherein the output power and wavelength of an optical signal to be output from the optical transmitter can be maintained constant with high accuracy.
In accordance with an aspect of the present invention, there is provided an optical transmitter comprising a light source for outputting a light beam; an external modulator for modulating the light beam output from the light source according to a main signal to output an optical signal; a power monitor for detecting the power of the optical signal output from the external modulator; and a first control unit for controlling the light source so that the power detected by the power monitor becomes constant.
With this configuration, the power of the optical signal output from the external modulator is detected, and the light source is controlled so that the power detected becomes constant. Accordingly, the power of the optical signal to be output from this optical transmitter can be maintained constant with high accuracy irrespective of variations in loss by the external modulator, thus achieving one of the objects of the present invention.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.